When an electronic ballast is used to drive more than one lamp, two types of circuit connections, i.e. series lamp circuit and parallel lamp circuit, are usually employed to connect the lamps. The parallel lamp circuits are widely used together with dimmable ballasts, with which the lamps may be dimmed. The main defects of parallel lamp circuits are that greater amounts of lamp currents pass through lamp inductors and an extra equalizer transformer is needed in the circuit. In practice, when a parallel lamp circuit is used for a dimmable ballast of more than one lamp, these defects may limit the application of the parallel lamp circuits.
A hybrid series-parallel lamp circuit is proposed to replace a purely parallel lamp circuit. FIG. 1 shows a schematic view of the circuit structure of a hybrid series-parallel lamp circuit for driving four lamps, where lamp 1 and lamp 2 are connected in series into a branch circuit, lamp 3 and lamp 4 are connected in series into another branch circuit, and these two branch circuits are connected in parallel. T2_1 and T2_2 are two windings of an equalizer transformer T2. An inductor Lr is connected to a high-frequency half-bridge circuit of an electronic ballast, which is not a critical part of the present invention and thus, not shown in the figures. Those two serially connected lamps are electrically asymmetric with respect to ground, The high frequency voltages of lamp 2 and lamp 4 drop at node 1 and node 2, respectively, forming hot points with respect to ground. Parasitic capacitive current Ipa passes parasitic capacitor Cpa, forming an AC path, as shown by dotted lines in FIG. 2, causing the amount of current passing lamp 1 and lamp 3 to be greater than that passing lamp 2 and lamp 4. From the point of view of vision, lamp 1 and lamp 3 are brighter than lamp 2 and lamp 4, in particular, when they are dimmed to below normal level, the difference in brightness will become more and more obvious, and finally, reach an extent that is unacceptable to users.
Another drawback of such a hybrid series-parallel lamp circuit is that, as shown in FIG. 3, when only three lamps are connected in the circuit, and account being taken of the fact that the voltages of the two branch circuits need to be balanced by the equalizer transformer T2, half of the lamp voltage in each branch circuit will drop on T2, causing the cold points in the circuit, i.e. the locations of node 3 and node 4 in FIG. 3, to be transformed to hot points and carry the high-frequency voltage of T2 and consequently, the parasitic capacitive current flows to ground. The current IT2_1 shown in FIG. 3 is equal to the vector sum of the lamp current Ila2 and the parasitic current Ipa, while the current IT2_2 is equal to the vector sum of the lamp current Ila3 and the parasitic current Ipa. Since there is a phase difference of 180° between the voltages of node 3 and node 4, the currents passing the two windings of the T2 are different. Although the current IT2_1 and IT2_2 are made as equal as possible through the equalizer transformer T2, the result of this balancing is not desirable, as T2 does not know the real current of the lamp, resulting eventually in different lamp currents Ila2 and Ila. Furthermore, also due to the existence of the parasitic current Ipa, the lamp current Ila1 is different from the lamp current Ila2; this is similar to the case of four lamps of FIG. 2.